Field of the Invention
This invention is related in general to the field of pumps for slurries. In particular, it relates to a centrifugal pump with a modified modular geometry that reduces wear and allows replacement of casing components to extend the service life of the pump.
Description of the Related Art
Mixtures of liquids and solids, such as slurries in mining and mineral processing operations, are typically moved using centrifugal pumps. The rotating impeller of the pump produces a pressure differential that moves the slurry from the axial input port to the radial discharge section of the pump. The centrifugal force generated by the impeller produces suction at the input port and causes the slurry to discharge at relatively high velocities with a radial component that produces abrasion on the inner wall of the peripheral portions of the casing.
In addition, slurry particles caught between the rotating impeller and the static walls of the casing produce wear on both components of the pump. This problem is more prevalent and critical on the suction side of the impeller because the high-pressure liquid in the discharge tends to flow toward the low-pressure zone in the suction section of the pump through the clearance between the rotating impeller and the static front casing wall. As the abrasion produced by such bypass flow widens this clearance, the amount of slurry recirculation increases and results in a loss of pump hydraulic performance and efficiency. Therefore, wear on the front suction side of the impeller is particularly undesirable. On the other hand, wear on the back side of the impeller is less significant because there is no bypass flow to the shaft side of the impeller.
As a result of this continuous abrasive action of the slurry on the impeller and the walls of the casing, slurry pumps ultimately fail and cause unintended shutdowns with attendant high economic losses. Therefore, periodic maintenance shutdowns are preferred and are regularly scheduled in order to minimize downtime. Typically, the life of the casing determines the ultimate length of service of a pump, but the liner at the suction side of the casing and the impeller need to be replaced one or more times at scheduled maintenance shutdowns during that time.
In order to reduce the wear caused by slurry particles moving between the impeller and the casing, expelling vanes have been used for decades in the clearance between the impeller and the casing walls around it. These vanes promote discharge of the particles and also reduce bypass recirculation, but this problem has persisted as a significant factor in causing undesirable downtime, whether planned or accidental.
The problem was addressed in U.S. Pat. No. 5,921,748, which disclosed a sealing arrangement that practically eliminates any augmentation in the clearance between the suction side of the impeller and the corresponding wall of the casing, thereby maintaining a relatively constant level of clearance and hydraulic performance during the life of the pump. An axially adjustable wear ring is added to the conventional pump configuration to substantially eliminate the clearance between the impeller and the casing wall in the suction zone of the pump. As the ring is worn over time, its position is adjusted by pushing it inward so as to maintain the appropriate seal with only sufficient clearance for the impeller to rotate freely with no significant bypass. In addition, in order to avoid wear caused by trapping, the '748 Patent also teaches an increase in the height of the expelling vanes and in the clearance between the vanes and the liner of the front suction wall of the casing beyond the largest particle size expected in the slurry.
While the seal and clearances taught in U.S. Pat. No. 5,921,748 represented a significant improvement in the art, long-term usage showed that additional wear problems remained unresolved. The wear and tear on the peripheral inner wall of the casing remains a critical limiting factor in the life of the pump casing. In addition, while bypass recirculation was dramatically reduced by the '748 invention by decreasing the pressure at the interface between the impeller and the wear ring, the higher expelling vanes and clearance between the impeller and the casing front wall proved to create localized increases in slurry turbulence that produce very high abrasion on the casing liner, both at its interface with the casing and its interface with the wear ring. As a result, the performance of the pump was improved materially, but wear and tear on the peripheral wall of the casing and at its interface with the front liner remains a problem and there is still a need for a pump-casing design that affords a service life commensurate with that of the other parts of the pump casing.